1. Field of the Invention
This invention relates to solid state current mirrors and more particularly to arrangements for providing a plurality of substantially equal currents from high impedance sources which are each controlled by at least one input control current.
2. Description of the Prior Art
Current sources characteristically provide a current from a very high impedance source. It is desirable that fixed current sources produce a constant output current the magnitude of which is independent of the impedance of the load which utilizes the current and variations in the voltage of the supply which energizes the current source. Such independence from voltage supply variations and load characteristics is also desirable in current sources of the type which are not fixed. The magnitude of output current from such nonfixed current sources should vary only in response to a control signal or an advantageously selectable circuit parameter such as the value of a resistance. Current mirrors are current sources the output current of which is controlled by an input current signal. Such circuits are well known and commonly used in applications where a fixed ratio is required between the relative magnitudes of the input and output currents. In an illustrative generalized current mirror arrangement the input and output currents bear a fixed relationship in the ratio n:m respectively. Thus, although varying amounts of current are drawn from a source of energizing potential, the output current will always be a fractional part m/n of the input current. This fraction may be viewed as the gain of the mirror, which gain may be less than unity in cases where the magnitude of the input current is greater than that of the output current. Current mirrors find many applications in electronic circuits, for example, they are used to supply current to various operational amplifier stages, and to convert differential stages to single-ended stages.
A first problem in the prior art to which this invention is directed relates to the precision of the fixed relationship between the magnitudes of the input and output currents in current mirrors. Assuming, for example, current mirror circuitry of the well-known Widlar type, which is further assumed to be comprised of transistor elements with matched common emitter current gains (.beta.), input and output currents which are desired to be equal to each other will deviate in proportion to the first power of g where g=1/(1+.beta.). Such inequality of input and output currents is generally the result of current merging and branching at circuit nodes within the particular circuit configuration. However, the prior art has improved the precision of the correlation between the input and output currents in some current mirror arrangements. One such improved current mirror arrangement which reduces input to output current deviation to a level proportional to g.sup.4 is disclosed in U.S. Pat. No. 3,936,725 which issued Feb. 3, 1976 to the above-named applicant.
The above notwithstanding, the prior art has not produced a current mirror arrangement which provides plural output or input control currents which deviate in magnitude from each other to a precision proportional to g.sup.4. One multioutput current source which is illustrative of prior art arrangements and exhibits a precision of current deviation in proportion to 1/.beta. is disclosed in U.S. Pat. No. 3,754,181.
A second problem in the prior art to which this invention is directed relates to producing an output current which bears a precise integer, but unequal, magnitude relationship to an input current. One prior art solution to the problem of achieving input to output current ratios (m:n) other than 1:1 requires the use of current mirror transistors which have unequal emitter-base junction areas. The current conducted through each transistor is proportional to its respective junction area. This technique for producing proportional current gain has the disadvantage of requiring the use of transistors having dissimilar geometries. Therefore, the current gain ratio is fixed at the time of manufacture and cannot be selectably altered thereafter.
It is, therefore, a general object of this invention to improve multioutput current sources of the type which provide output currents responsive to an input control current signal.
More specifically, it is an object of this invention to provide multioutput current mirror arrangements which exhibit improved magnitude correlation between the input and output currents over prior art current mirrors.
It is a further object to provide multioutput current sources which have higher output impedance than prior art arrangements so as to permit selectable paralleling of the output current terminals without unacceptable output impedance degradation.